Characterization of a natural magnetite

Single crystals of a rock magnetite were separated from steatite cobbles collected in a geological site near the city of Serro (18degrees 36' 47'' S 43degrees 22' 46'' W), Minas Gerais, Brazil. A typically well-shaped magnetite single crystal was characterized by chemical analysis, Fe-57 Mossbauer spectrometry at 300, 77 and 4 K and under an applied magnetic field of 6 T at 10 K, magnetization measurements and electronic microprobe. From Mossbauer data, the sample is stoichiometric with a tetrahedral and octahedral site occupancy ratio of 1:2. Elemental chemical analysis and point-to-point electron microscope probing show some inclusions of lamellar ilmenite (less than or equal to 1 mass%) randomly distributed throughout the magnetite matrix, and also that the magnetite matrix is constituted only by Fe2+ and Fe3+, with no isomorphic substitution. Results are discussed on the basis of the magnetization curve and of the temperature dependence of the AC magnetic susceptibility. The Verwey transition occurs in the temperature range of 100-115 K, observed by a sudden change in the temperature dependence of the magnetization.